The control group participants of the current study were ver
The control group participants of the current study were very similar in age to those of Puertas et al. There was no significant correlation between age and enzyme activity over the short age span studied although age-related changes in the activity of ApA and aspartyl aminopeptidase, typically increasing activity with age, have been reported over the life-span (Martinez et al., 1998, Ijima et al., 2002). Similarly the current study did not detect any gender differences in enzyme activity in the control group nor the combined control and patient group, this is at variance with Puertas et al. (2013) who identified a significantly lower activity of ApA in control females than control males. Martinez et al. (1998) also previously reported decreased activity of ApA and aspartyl aminopeptidase in females compared to males, but only in the 16–45 and 46–65year age groups respectively. Hypertension and use of antihypertensive agents was not an exclusion criterion of the current study, partly because of the ubiquitous nature of the condition. Hypertension is known to be associated with increased ApA activity (Ijima et al., 2002), and any form of long-term therapy acting on the RAS might be expected to alter enzyme activity. Such lack of exclusion may explain the failure to replicate the decrease in IRAP function in the current study, but the replication of the findings of the earlier study for ApA, ApB and ApN again suggest that the earlier findings of Puertas et al. can be generalised to a more heterogeneous population. Within the literature there is some disagreement concerning the specificity of the enzymes studied, and thus the appropriate choice of substrate. In all cases, the current study utilised substrates comparable to those used by Kuda et al. and Puertas and colleagues, the findings are therefore comparable. ApA is the generally recognised name for glutamyl-aminopeptidase (EC220.127.116.11) although typically it is also seen as being responsible for the conversion of angiotensin II to angiotensin III by the removal of aspartic acid, there is therefore some confusion between the activities of ApA and aspartyl-aminopeptidase (Mayas et al., 2001). Activity of ApA was determined by the removal of glutamic BKT140 from glutamyl-β-naphthylamide and glutamyl-p-nitroanilide in the earlier and current study respectively, thus there can be confidence concerning consistency of the findings. IRAP is perhaps the most complicated of the aminopeptidases being investigated. Originally identified as oxytocinase or human placental leucine aminopeptidase (PLAP), it is known to cleave leucine and cystine moieties. It is believed to be involved with the degradation of endogenous oxytocin, vasopressin, lys-bradykinin, met-enkephalin, dynorphin A and other peptide hormones, although bradykinin and met-enkephalin contain neither leucine nor cystine, indicating its lack of specificity. IRAP has also been shown to be the active binding site for angiotensin IV (Albiston et al., 2001), with the enzyme activity being inhibited by angiotensin IV. Aminopeptidase activity was determined by cleavage of leucyl-β-naphthylamide and leucine-p-nitroanilide in the two studies respectively. Two other differences between the current study and that of Puertas et al. (2013) were the use of serum rather than plasma, and the expression of enzyme activities relative to serum volume or plasma protein. Whilst for some proteins there are markedly different concentrations in serum and plasma, for example brain derived neurotrophic factor (240 amino acids) for which concentrations are 10-fold higher in serum than plasma (e.g. D'Sa et al., 2012), the aminopeptidases (e.g. ApN: 967 amino acids, IRAP: 1024 amino acids) do not appear to differ in concentration between the two fluids (e.g. Fylling, 1964, Riad, 1966). Total plasma protein concentrations, however, do differ between serum and plasma (Miles et al., 2004), with total protein approximately 2.5% lower in serum. Clinical chemistry results are typically reported as quantity or activity per unit volume with protein concentrations allowing some potential correction for hydration or nutritional status but as there were no significant differences in serum protein concentration between the diagnostic groups, such correction was deemed redundant. The results indicated that when correction for serum protein concentration was applied, some of the previously identified significant differences in enzyme activity between the groups were lost. It is our belief that use of serum without correction for protein concentration is unlikely to have significantly compromised the replication of the earlier study, other than to have obscured some of the differences previously identified, possibly due to the marginal difference in protein concentration between serum and plasma.